Effect of Magnetic Field on Microstructure Evolution during Disorder-Order Transformation in an Fe-Pd Alloy

Abstract

Control of microstructure through solid-solid phase transformation under magnetic field is a promising method to improve the properties and design new materials. In general, solid-solid phase transformations are classified into two groups from a viewpoint of atomic diffusion: diffusionless (martensitic) transformations and diffusional transformations. The characteristics of both diffusionless and diffusional transformations are known to be influenced by external fields such as magnetic field and pressure. In the case of the martensitic transformations, the effects of magnetic field on microstructure have been intensively studied. A well-known example is the rearrangement of martensite variants driven by magnetic field in ferromagnetic shape memory alloys. In these alloys the variant with the lowest magnetocrystalline anisotropy energy is selected to grow consuming others by twinning deformation, and the condition for realizing the rearrangement has been derived quantitatively. 1–4) On the other hand, concerning diffusional transformations, the effect of magnetic field on microstructure has not been so clarified yet although many interesting results have been reported until now. 5–10) Considering the effectiveness of magnetic field on selection of variants in ferromagnetic shape memory alloys, we can also expect a selective formation of a variant which is formed in a diffusive process in alloys with a large magnetocrystalline anisotropy. As for such materials, Co-Pt and FePd alloys can be a suitable system. The reason is as follows. The disorder-order transformation in CoPt and FePd is one of the representative diffusional transformation in which the microstructure formation is expected to be influenced by magnetic field because of their large magnetocrystalline anisotropies. The structure change associated with this disorder-order transformation is from an A1-type (cubic) disordered structure to an L10-type (tetragonal) ordered structure and the ordered phase has three lattice corresponding variants and is characterized by a high uniaxial magneto